Alterations in vascular permeability are a defining feature of diverse processes including arteriosclerosis, inflammation, ischemia/reperfusion injury, angiogenesis, and the subject to intense investigation. In contrast, there is little known about processes that determine barrier protection or barrier restoration after edemagenic agents. Platelet-derived products are essential to maintaining the integrity of the endothelial cell barrier although this mechanism of barrier protection is undefined. In published data relevant to this Project, we demonstrated that sphingosine 1-phosphate (Sph 1-P), a phosphorylated lipid angiogenic factor released from activated platelets, ligates specific endothelial differentiation gene (Edg) receptors to stimulate chemotaxis and angiogenic responses. Sph 1-P produces rapid, sustained and dose-dependent increases in vascular barrier integrity in vitro and in vivo, and reverses barrier dysfunction elicited by thrombin. The mechanisms by which Sph 1-P enhances barrier function are unknown, however, our data strongly implicate endothelial cell cytoskeletal dynamics in this response. Sph 1-P-mediated barrier enhancement is depending upon cortical actin filament rearrangement and Rac-dependent recruitment of known cytoskeletal regulatory proteins. In this Project, we will examine the molecular basis of Sph 1-P-induced barrier enhancement and have targeted cortical cytoskeletal interactions with membrane adhesive proteins that promote vascular integrity. SA#1 will characterize the Sph 1-P-mediated rearrangement of the cortical cytoskeleton following shear stress and cyclic stretch and assess PAK, LIM kinase cofilin and PKCdelta involvement. SA#2 will investigate the role of Sph 1-P-induced tyrosine dephosphorylation in the alterations of zonula adherens (catenin complex) interaction with the actin cytoskeleton. SA#3 will define the involvement of the actin-binding protein, cortactin, in cortical actin rearrangement. SA#4 will define the involvement of the activated p125 focal adhesion kinase and Rho/Rac GTPase in Sph 1-p-medicated regulation of focal adhesion structure/function. Given the profound physiologic derangements that accompany vascular leak in multiple vascular pathobiologies, Sph 1-P may provide a novel therapeutic intervention for treatment of these devastating disorders.